A. Field of the Invention
The present invention relates to automated steering of vehicles and, in particular, to the automatic steering of articulated vehicles.
B. Problems in the State of the Art
Differently from front wheel-steered vehicles, articulated vehicles can change their heading rapidly via articulation angle, pivoting (steering) mechanism placed almost in the middle of the vehicle (see FIG. 1G-I). At low speed and even at standing still they can change vehicle heading by pivoting. However, this is not desired due to jerky rotation (jerky lateral motion). Besides, instant quick responses to heading changes can result in cross track error, lateral movement of the control point from the desired path depending on the selection of control point on the vehicle. And more importantly, these quick heading changes can easily lead to bouncing and eventually to oscillatory response in steering system and in path following. It is known that main advantages of articulated vehicles are traction and power superiority. Therefore, a practical approach to alleviate or remove the jerky heading change is to cancel the rate of change of articulation angle in heading kinematics. Besides, this modification (also called a transformer) can be done such that articulated vehicles can change their heading similar to front wheel-steered vehicles. Here, we picked the parameters in design such that articulated vehicles with respect to heading kinematics will behave like a commercially-available front-wheel steered vehicle (i.e. Case 210) as a nominal vehicle. This approach also allows the use of the gains, tuned for a Case 210, for articulated vehicles. This eliminates an important amount of time of tuning. As will be appreciated by those skilled in this technical area, the designer according to the invention can select parameters or other nominal vehicles.
Automatic steering communicates a programmed path for the vehicle to travel to an actuator that changes the wheel angle of the vehicle at one or more axles. In many agricultural vehicles, a hydraulic system translates steering instructions from a steering wheel or other manual control to steerable wheels. See FIG. 1A for one basic example. Autosteering uses programmable devices to directly control such actuators.
In agriculture, path instructions are typically relative to a desired wheel track or path through a field. One example is a continuous path through an entire field for effective and efficient spacing of row crops. This can involve multiple turns, such as row ends, and non-linear rows, which also requires steering. Another example is following planted row crops when applying chemicals such as fertilizer in a non-overlapping manner.
Autosteering systems cooperate with navigation systems which use sensors to estimate such things as speed and position in the field versus intended path. A variety of commercially available autosteering/navigation systems for agricultural vehicles exist. One example is SteerCommand™ from Ag Leader Technology, Inc. of Ames, Iowa USA.
Generally, autosteering relies on steering angle instructions from a navigation planner. See FIG. 1B for a high-level schematic of the type of automated steering system to which the invention can be applied.
The navigation planner relies on sensor measurements from which such things as vehicle position/speed, heading, yaw, etc. can be estimated. See. FIG. 1B and FIG. 1C, which are block diagrams of examples (Parts 1 and 2) of an automated steering system to which the invention can be applied.
Many of these autosteering systems use what is called a PID type controller. See PID element in context of autosteer system in FIG. 1C and simplified diagram of PID operation at FIG. 1D for illustration of this well-known technique. In FIG. 1C (part 1), the outer loop is for the cross track error, XTE and the inner loop is for the heading error, ΔΨ. They are shown on a farming operation in FIG. 1F. In FIG. 1C (part 1), blocks D1 and D2 stand for any unit conversion or constants, etc., used in the loops. In FIG. 1C (part 2), the vehicle block is explored, where steering system and estimation system, Kalman Filter, are seen. IMU stands for inertial measurements unit, accelerometers, gyros, and possibly magnetometer.
PID control can help produce inputs that reduce offset between commanded and measured actual signals. This signal can be steering angle, heading angle, lateral displacement in the field. More details on PID control relative to autosteering can be found at U.S. Pat. No. 7,142,956, incorporated by reference herein.
A block diagram of a PID controller in a feedback loop is set forth below at FIG. 1D, where r(t) is the desired process value or “set point”, and y(t) is the measured process value (here, steering instructions):                a. P accounts for present values of the error. For example, if the error is large and positive, the control output will also be large and positive.        b. I accounts for past values of the error. For example, if the current output is not sufficiently strong, the integral of the error will accumulate over time, and the controller will respond by applying a stronger action.        c. D accounts for possible future trends of the error, based on its current rate of change.        
This PID control is a fairly mature technique in the industry and widely used. It is based on well-known equations and inputs. While it works fairly well for its intended purpose, several competing factors make room for improvement in this area.
For example, sometimes a vehicle does not move in a precise intended path in the field. Field conditions (e.g. dry or wet, uneven ground, debris, and other things) can cause deviations. Additionally, the type of vehicle, how is driven, and its traction can cause deviations including such well-known issues as cross track error (XTE). A rear wheel drive, front wheel steered vehicle (FrV) will tend to understeer or slide relative a programmed path. This could require quite substantial and severe steering correction.
On the other hand, even quick and aggressive steering corrections to try to correct error between the actual heading and the programmed heading may not produce the intended result. Such things as sliding of the front of the tractor, cross track error, or the like, require the PID controller to adjust steering control to compensate for these types of complexities.
PID based steering controllers try to balance these sometimes competing factors by using one, two, or more PID compensators to minimize offset between the programmed path and measured path. Each of them can be tuned for even more control, such as is well-known in this technical area. Front wheel steered vehicles (FrV) such as the Case Model 210 (see FIG. 1E) utilize a navigation nomenclature such as set forth in FIG. 1F.
But increasing popularity of articulated tractors (ArV's) (see, e.g., both wheeled and tracked ArV's at FIG. 1G and FIG. 1H) has complicated the situation. As mentioned above, superior traction and power enable larger payloads and wider implements with ArV's. However, the articulated vehicles, by nature, tend have more responsive steering control than front steer vehicles. The increased traction produces quicker and more faithful response to a steering adjustment. But this can lead to jerking or other disruptive motions.
A basic simplified plan view of such a vehicle and its navigation coordinates is shown at FIG. 1I. This shows some of the complexities articulated vehicles present relative to autosteering.
Another schematic of those navigation coordinates is shown below at FIG. 1J, and will be used in later descriptions of how to implement the invention.
With increased responsiveness an ArV has to steering instructions comes a problem. Consider, first, a FrV. Turning the front wheels while the rear driven wheels operate, attempts to push the front of the vehicle in a straight line. Some lag, cross track error, and other complications are created. However, these can be compensated by the navigation system and the PID compensation.
Compare an ArV. Because it at least pivots along this frame, and typically has front and rear driven wheels, the understeer or rear driven wheels problem is lessened. But the improved traction and power tend to produce quicker steering response, including for substantial steering changes.
One problem becomes a jerky, jostling action by an articulated vehicle in response to autosteering. Because autosteering attempts to follow as precisely as possible the programmed path, this can range from being merely unsettling to the operator to being quite disruptive to the operator. It also may actually increase wheel deviation from the programmed path with such things as cross track error. These types of problems are explained in U.S. Pat. Nos. 4,756,543; 4,103,561; and 7,124,579, each of which is incorporated by reference herein in its entirety respectively.
A subtle complexity is that if the autosteering system is set up for front steer vehicles, tuning the PID controller to compensate for an articulated vehicle may not be sufficient to eliminate disruptive autosteering. Furthermore, it can be desirable to use the same steering system for a variety of vehicles, both front steer and articulated, as well as get the same operator “feel” of FrV response with ArV's.
Thus, there is room for improvement in this technical field. The subtlety is how to compensate for disruptive motion of autosteering with an articulated vehicle.